This invention relates to the catalytic hydrotreatment of coal-derived liquids and similar hydrocarbon materials. More particularly, this invention relates to the use of a catalyst comprising a carbide and nitride of a Group VIB metal in such hydrotreating processes.
In the petroleum refining industry, hydrotreating processes are in wide use. Typical applications range from mild hydrosweeting carried out at high space velocities, low pressures and temperatures, and minimal hydrogen consumption. The object of these processes is to remove sulfur, nitrogen, and other non-hydrocarbon components, as well as to improve odor, color, stability, and other important quality characteristics. A more severe hydrotreating process includes the saturation of aromatic compounds present within heavy oils. This is useful for producing clean-burning fuels, improving the octane of diesel fuels, and reducing the coking propensity of stocks fed to catalytic cracking units for further upgrading. The most severe hydrotreating processes are used to convert residual feedstocks with atmospheric boiling points exceeding 1,000.degree. F. In each of these applications, the process is carried out in the presence of a catalyst.
Catalytic hydrotreating is not limited to feedstocks derived from petroleum. Because the catalytic chemistry of hydrotreating petroleum-derived hydrocarbons and hydrocarbons derived from oil shale, coal, and tar sands is similar, the same processes and catalysts are used to produce synthetic fuels. Indeed, many of the current petroleum processes and catalysts originated from research on coal liquefaction conducted in Germany in the 1930s and '40s. Coal liquefaction was used to supply fuels for the German war effort in World War II. The production of synthetic fuels remains topical today as a means to reduce United States's dependence on foreign crude oil.
Generally, catalysts employed for hydrotreating processes are comprised of composites of Group VIB or Group VIII metal hydrogenating components, or both, with an inorganic oxide base, or support, typically alumina. These catalysts typically are treated with sulfur-containing compounds to activate the Group VIB and Group VIII metals by converting them to their respective sulfides. While these catalysts have been effective there is a need for improvements. Recent concerns about the environment have necessitated a re-evaluation of all of the petroleum refining industry's hydrotreating processes. Furthermore, breakthroughs in hydrotreating catalyst technology would have a major impact on the economics of producing synthetic fuels.
A preferred alternative hydrotreating catalyst would contain metals from non-Group VIII because the latter are amongst the most expensive catalytic metals. Novel catalytic properties would then be imparted by the incorporation of inexpensive elements such as carbon and nitrogen. In the past, carbides and nitrides of Group VIB metals were not used as hydrotreating catalyst because of problems in synthesizing these catalyst with high surface areas. Recently, preparation techniques were developed to prepare high surface area carbides and nitrides of Group VIB metals. U.S. Pat. No. 4,515,763 discloses a method of preparing high surface area carbides and nitrides of Group VIB metals. In this patent, bulk carbides and nitrides of Group VIB metals having a surface area of up to 188 m.sup.2 /g are disclosed. Other methods of preparing high surface area carbides and nitrides of Group VIB metals are disclosed in U.S. Pat. Nos. 4,325,842, 4,325,843, and Lee, J. S., Oyama, S. T., and Boudart M. "Molybdenum Carbide Catalyst", Journal of Catalysis 106, 125-133 (1987). None of these references disclose or suggest using these catalyst as a hydrotreating catalyst.
The use of carbides and nitrides of Group VIB metals as a hydrotreating catalyst for certain model feeds has been disclosed in the prior art. In the paper Schlatter, James C., Oyama, S. Ted, Metcalfe, III, Joseph E., and Lambert, Jr., Joseph M., "Catalytic Behavior of Selected Transition-Metal Carbides, Nitrides, and Borides in the Hydrodenitrogenation of Quinoline," Industrial Engineering Chemistry Research, Vol. 27, p. 1639 (1988), high surface area carbides and nitrides of Group VIB metals were synthesized and tested for hydrodenitrogenation activity with quinoline. The experiment reported on by the Schlatter paper was conducted at a pressure of only 1,000 psig. In addition, quinoline boils at 458.degree. F. and contains only 17 atoms. The feedstock and hydrotreating conditions disclosed in the Schlatter paper were substantially different than typical commercial hydrotreating processes which can have a pressure of up to 2,000 psig and feedstocks containing materials boiling greater than 1,000.degree. F. In Lee, J. S., Boudart, M., "Hydrodesulfurization of Thiophene Over Unsupported Molybdenum Carbide," Applied Catalysis Vol. 19 (1985) 207-210, high surface area unsupported molybdenum carbide was synthesized and tested for hydrosulfurization of thiophene. The experiment reported on by the Lee paper was conducted at a pressure of only 15 psig. Moreover, thiophene, which boils at 183.degree. F. and contains only 9 atoms, has a high desulfurization rate in comparison with higher boiling sulfur compounds typically found in commercial hydrotreating feedstocks and, therefore, can be successfully desulfurized using catalyst and process conditions that would be ineffective for desulfurizing commercial hydrotreating feedstocks.
The size of the constituent molecules in commercial hydrotreating feedstocks is important. The molecules must be sufficiently small to enter the pores of the hydrotreating catalyst. While the pores of catalysts similar to the catalysts described in the Schlatter and Lee papers above are of sufficient size to admit small molecules, such as quinoline and thiophene, these catalysts are not expected to admit the much larger molecular constituents of commercial hydrotreating feedstocks.
It has been suggested that carbides and nitrides of Group VIB metals are not suitable for heavier hydrocarbon feedstock due to high sulfur content of such feedstocks. In the Schlatter paper, molybdenum carbide was tested with quinoline in the presence of sulfur. The results were a decrease in both hydrodenitrogenation activity and selectivity in comparison with a conventional Ni-Mo alumina catalyst. Further, in Burton, James J., Garten, Robert L., Advanced Materials in Catalysis, Academic Press, 101, (1977), it is disclosed that carbides of transition metals are unstable in high H.sub.2 S concentrations, particularly the level of H.sub.2 S typically found in coal liquids. Sulfur poisoning of carbide and nitride catalysts is even more likely to occur at commerial hydrotreating conditions because the high pressures lead to high partial pressures of H.sub.2 S.
There is a need for an improved hydrotreating process using carbides and nitrides of Group VIB metals. More specifically, there is a need for a process using carbides and nitrides of Group VIB metals for effective hydrotreating of heavier, high sulfur-containing hydrocarbon feedstocks at commercial conditions.